Temperature control is a process in which change of temperature of a space, object, liquid, or gas is measured or otherwise detected, and the application or release of heat energy to the space, object, liquid or gas is adjusted to achieve a desired average temperature. Temperature control is important to a variety of equipment and processes, such as the cooling of an internal combustion engine or the temperature of an oven employed to cure a coating or adhesive.
One goal of a temperature control system is to maintain a temperature within a specified range. A temperature control system includes a mechanism for adding or removing heat, a temperature sensing device, and means for actuating the mechanism for adding or removing heat that is responsive to the temperature sensing device. Time delay is inherent in the addition or removal of heat. Temperature sensing devices have some variability. Thermo-mechanical temperature control devices typically respond differently to an increase in temperature than to a drop in temperature, with the difference between the responses referred to as “hysteresis.” Mechanically actuated electric switches also have hysteresis between a change of state during actuation and release. Hysteresis in a temperature control system produces different temperatures for control inputs during heating and cooling, resulting in two temperature set points, rather than a single temperature set point. A prior art temperature control switch may employ various means to reduce hysteresis in a temperature control system, to increase the accuracy of temperature control.
Hysteresis is further defined as the lag between an input and an output within a system, based upon previous and instantaneous inputs. In a basic, open loop temperature control, a hysteresis band can be employed such that an actual temperature of a given area is used as an input signal and a change from a first state to a second state of the temperature control is the output. In this system, hysteresis is the difference between the temperature at which a control switch is in a first state and the temperature at which the control switch is in a second state. In a more sophisticated, closed loop temperature control, such as a digital temperature control, the actual temperature is constantly measured by a sensor and feedback is sent to a controller to adjust an output value.
Some processes or systems may require widely separated temperature set points. For these processes or systems, two separate single set point control switches are typically required. In such a system, there is a first switch having a first set point and a second switch having a second set point, which allows for a desired output from two separate set points. However, using two temperature control switches, or a digital temperature control switch, is a less efficient use of space, may require an external power source, wired connections, more moving parts, increased complexity, and includes more modes of failure, all of which make using two temperature control switches, or a digital temperature control switch, more costly than using one basic temperature control switch.
There is a need in the art for a simple, low-cost temperature control switch with few moving parts, no external power source, and few modes of failure that can reliably change states at two widely separated set points.